They blend & if you let it sit idle for weeks, then it's likely that it would separate. Think of it this way: why doesn't E85 separate in the underground tanks at gas stations? It's cause anytime somebody pumps a couple gallons, the entire mixture gets agitated (maintaining an equal blend).

They blend & if you let it sit idle for weeks, then it's likely that it would separate. Think of it this way: why doesn't E85 separate in the underground tanks at gas stations? It's cause anytime somebody pumps a couple gallons, the entire mixture gets agitated (maintaining an equal blend).

I thought it was mixed at the pump? Maybe that isn't all the time... hmmm.

After talking with my girlfriend (chemist) and doing some more reading, I think the biggest problems are going to be temperature and water content.

As the temperature decreases, the amount of ethanol that is able to be in solution with the gasoline will decrease. As water content increases, the polar nature of ethanol will cause it to be attracted to the water, which causes the phase separation you hear about concerning ethanol. The amount of water that can be present before phase separation occurs drops with temperature as well. Once phase separation has occurred, it cannot be undone without separating the gasoline from the phase separated ethanol, distilling the ethanol and adding it back to the gasoline. Ethanol is hydrophilic, so it will attract water from the air.

After talking with my girlfriend (chemist) and doing some more reading, I think the biggest problems are going to be temperature and water content.

As the temperature decreases, the amount of ethanol that is able to be in solution with the gasoline will decrease. As water content increases, the polar nature of ethanol will cause it to be attracted to the water, which causes the phase separation you hear about concerning ethanol. The amount of water that can be present before phase separation occurs drops with temperature as well. Once phase separation has occurred, it cannot be undone without separating the gasoline from the phase separated ethanol, distilling the ethanol and adding it back to the gasoline. Ethanol is hydrophilic, so it will attract water from the air.

After reading the study linked to above authored by Bridgeman and Querfeld, four factors come into play concerning ethanol in solution with gasoline:

1. The temperature of the solution. Lower temperatures results in a higher likelihood of separation.

2. The water content of the solution. More water results in a higher likelihood of separation.

3. The concentration of ethanol to gasoline. More ethanol results in a LOWER likelihood of separation

4. The gasoline used. Different crude makes different gas, and this has an effect on the ability of gas to hold ethanol (and water if present) in solution.

The third point was particularly interesting. Basically, a mixture of E10 would be more likely to separate than a mixture of E35. Considering the ethanol added to gasoline is anhydrous to begin with, I think it is safe to conclude that if E10 doesn't separate, then E35 won't either, all else being equal.

The study, from 1933, does note that additional additives to gasoline may decrease the likelihood of separation for a given amount of ethanol and water in solution with the gas. Gas formulations have surely changed since 1933, and probably have an ability to keep more ethanol in solution due to additives versus gas from 80 years ago, before E10 was widely used.

I think the data, notably the fact that E10 will separate at a lower temperature than a solution with a higher ethanol percentage, proves that if you feel safe letting your car sit with E10 in the tank, a tankful of E35 should only be less likely to separate.

The only problem would be water. ASTM D 4806 limits water in the ethanol used for motor fuels to 1% by volume. I've read that water contamination is not really a modern problem because gas distributors have corrected storage tank problems leading to water getting into the tank. Furthermore, even if the ethanol contained 1% water, it is mixed with 15% gas at E85, and then the ethanol percentage is further reduced when mixed with gas to achieve E30 or whatever is used in your car. You can be sure that the gasoline will not have any water in it, so a mixture of 99% ethanol and 1% water (the maximum allowable by ASTM D 4806) yields a sample with only 0.5% water at an E50 concentration, and lower water percentages for lower ethanol concentrations (E30 for instance).

If you are concerned about a given sample of an ethanol and gasoline solution, put it in the freezer. Residential freezers are supposed to maintain at or below 0 F. Most overnight low temperatures are not even this low, so if it doesn't separate in the freezer, it won't in your car either. If you suspect separation, a simple test using water soluble food coloring will reveal if water is the culprit. If water is present, the coloring will go into solution with it. If water is not present, the coloring will not mix at all, and should fall to the bottom of the sample. The sample may need mixed after the coloring is added, and then settling time may be necessary as well. I have never tried this personally.

In conclusion, separation of the ethanol and gas blend in your fuel tank is not likely, but the chance of separation increases in colder temperatures and with more water in the fuel. The chance of separation decreases as the percentage of ethanol in the solution increases. When in doubt, put a sample in the freezer, and if it appears cloudy after cooling, add water soluble food coloring to confirm the presence of water. However, water contamination is not really a problem any more, so you should be good under more conditions.

This is my analysis and is not the word of God. If you see a mistake, lets talk about it. However, this information reflects the knowledge I have gained from reading about the use of ethanol in motor vehicles, and is accurate as far as I know.

After reading the study linked to above authored by Bridgeman and Querfeld, four factors come into play concerning ethanol in solution with gasoline:

1. The temperature of the solution. Lower temperatures results in a higher likelihood of separation.

2. The water content of the solution. More water results in a higher likelihood of separation.

3. The concentration of ethanol to gasoline. More ethanol results in a LOWER likelihood of separation

4. The gasoline used. Different crude makes different gas, and this has an effect on the ability of gas to hold ethanol (and water if present) in solution.

The third point was particularly interesting. Basically, a mixture of E10 would be more likely to separate than a mixture of E35. Considering the ethanol added to gasoline is anhydrous to begin with, I think it is safe to conclude that if E10 doesn't separate, then E35 won't either, all else being equal.

The study, from 1933, does note that additional additives to gasoline may decrease the likelihood of separation for a given amount of ethanol and water in solution with the gas. Gas formulations have surely changed since 1933, and probably have an ability to keep more ethanol in solution due to additives versus gas from 80 years ago, before E10 was widely used.

I think the data, notably the fact that E10 will separate at a lower temperature than a solution with a higher ethanol percentage, proves that if you feel safe letting your car sit with E10 in the tank, a tankful of E35 should only be less likely to separate.

The only problem would be water. ASTM D 4806 limits water in the ethanol used for motor fuels to 1% by volume. I've read that water contamination is not really a modern problem because gas distributors have corrected storage tank problems leading to water getting into the tank. Furthermore, even if the ethanol contained 1% water, it is mixed with 15% gas at E85, and then the ethanol percentage is further reduced when mixed with gas to achieve E30 or whatever is used in your car. You can be sure that the gasoline will not have any water in it, so a mixture of 99% ethanol and 1% water (the maximum allowable by ASTM D 4806) yields a sample with only 0.5% water at an E50 concentration, and lower water percentages for lower ethanol concentrations (E30 for instance).

If you are concerned about a given sample of an ethanol and gasoline solution, put it in the freezer. Residential freezers are supposed to maintain at or below 0 F. Most overnight low temperatures are not even this low, so if it doesn't separate in the freezer, it won't in your car either. If you suspect separation, a simple test using water soluble food coloring will reveal if water is the culprit. If water is present, the coloring will go into solution with it. If water is not present, the coloring will not mix at all, and should fall to the bottom of the sample. The sample may need mixed after the coloring is added, and then settling time may be necessary as well. I have never tried this personally.

In conclusion, separation of the ethanol and gas blend in your fuel tank is not likely, but the chance of separation increases in colder temperatures and with more water in the fuel. The chance of separation decreases as the percentage of ethanol in the solution increases. When in doubt, put a sample in the freezer, and if it appears cloudy after cooling, add water soluble food coloring to confirm the presence of water. However, water contamination is not really a problem any more, so you should be good under more conditions.

This is my analysis and is not the word of God. If you see a mistake, lets talk about it. However, this information reflects the knowledge I have gained from reading about the use of ethanol in motor vehicles, and is accurate as far as I know.

I've mixed 93 and e85 (e30 for Cobb OTS race maps) mix and when I get to 2-3 gallons left in the tank I start throwing fuel codes. So now, I have to remind myself to either switch maps or throw more mix when I get below 4 gallons.

Can anybody explains why this happens? The fuel in the tank is supposed to be mixed but obviously something isn't right. Again it only happens when fuel gets low and I ran the tank low a few times just to verify.

I know there was a big debate over on the other forum (maybe it was this one) about the flex fuel monitor, but to me, this is where such a device would shine.

I've mixed 93 and e85 (e30 for Cobb OTS race maps) mix and when I get to 2-3 gallons left in the tank I start throwing fuel codes. So now, I have to remind myself to either switch maps or throw more mix when I get below 4 gallons.

Can anybody explains why this happens? The fuel in the tank is supposed to be mixed but obviously something isn't right. Again it only happens when fuel gets low and I ran the tank low a few times just to verify.

I know there was a big debate over on the other forum (maybe it was this one) about the flex fuel monitor, but to me, this is where such a device would shine.

Which fuel codes did you usually throw when you are low? Because I usually wait until its low as well and I don't throw fuel codes...